JavaScript Variables

Variables are Containers for Storing Data

JavaScript Variables can be declared in 4 ways:

• Automatically
• Using var
  
• Usinglet
  
• Using const
  

In this first example, x, y, and z are undeclared variables.

They are automatically declared when first used:

x = 5;
y = 6;
z = x + y;

Note
It is considered good programming practice to always declare variables before use.

From the examples you can guess:

• x stores the value 5
• y stores the value 6
  
• z stores the value 11
  

var x = 5;
var y = 6;
var z = x + y;

Note
The var keyword was used in all JavaScript code from 1995 to 2015.

The letand const keywords were added to JavaScript in 2015.

The var keyword should only be used in code written for older browsers.

let x = 5;
let y = 6;
let z = x + y;

const x = 5;
const y = 6;
const z = x + y;

const price1 = 5;
const price2 = 6;
let total = price1 + price2;

The two variables price1 and price2 are declared with the const keyword.

These are constant values and cannot be changed.

The variable totalis declared with the let keyword.

The value total can be changed.

When to Use var, let, or const?

1. Always declare variables

2. Always use const if the value should not be changed

3. Always use const if the type should not be changed (Arrays and Objects)

4. Only use let if you can't use const

5. Only use var if you MUST support old browsers.

Just Like Algebra

Just like in algebra, variables hold values:

let x = 5;
let y = 6;

Just like in algebra, variables are used in expressions:

let z = x + y;

From the example above, you can guess that the total is calculated to be 11.

Note
Variables are containers for storing values.

JavaScript Identifiers

All JavaScript variables must be identified with unique names.

These unique names are called identifiers.

Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).

The general rules for constructing names for variables (unique identifiers) are:

• Names can contain letters, digits, underscores, and dollar signs.
• Names must begin with a letter.
  
• Names can also begin with $ and _ (but we will not use it in this tutorial).
  
• Names are case sensitive (y and Y are different variables).
  
• Reserved words (like JavaScript keywords) cannot be used as names.
  

Note
JavaScript identifiers are case-sensitive.